Approximately 70% of the Earth's surface is covered by water. However, only a small fraction of the world's water supply is naturally accessible as freshwater for household, agricultural, industrial, environmental and other purposes. The primary source of freshwater is currently atmospheric precipitation, mostly as rainfall or snowfall, which is captured in surface and underground structures and formations such as lakes, reservoirs, aquifers, watersheds, snowpack and glaciers.
Significantly, recent changes in climactic conditions have led to reduced atmospheric precipitation in many geographical areas where freshwater has historically been captured or utilized-resulting in severe droughts, depletion of supply, environmental harm, excess demand and other undesired consequences. Moreover, increasing population in these and other locations has accelerated the effects of reduced freshwater availability and contributed to a growing need for additional or alternative sources of freshwater.
Traditional responses to shortages and fluctuations in freshwater supply have included conservation, rationing, recycling and wastewater treatment, among others. While these approaches may be helpful in providing near term adjustments to changing conditions, they do not increase the overall supply of freshwater. Other conventional solutions have included construction of large dams, reservoirs and other facilities for capture and storage of freshwater, but such solutions are dependent upon favorable climactic conditions and limited by geological, structural and other factors. Additionally, freshwater has been transported from particular resources or regions to others through pipelines, aqueducts, tunnels and canals, but these methods generally diminish overall supply through evaporation, seepage and other loss, and may result in environmental or other harm to areas where water is removed, diverted or transported.
Desalination of seawater or brackish water has also been utilized to provide additional or alternative supplies of freshwater. The primary methods of desalination have generally been distillation or filtration, which typically require substantial energy consumption and complex facilities and infrastructure. As an example, multi-stage flash distillation (MSF) is a thermal process that involves heating and sequential flash evaporation of feed water through a series of stages or chambers having lowered vapor pressures at successive stages. The evaporated feed water is condensed at each stage to form product water having reduced salinity and the remaining feed water is discharged as brine. A similar process to MSF is multiple-effect distillation (MED) which utilizes evaporated steam to heat feed water in successive stages through a series of connecting tubes. When feed water inside a chamber contacts a connecting tube filled with steam, the feed water partially evaporates and the steam inside the tube condenses to form product water. The remaining feed water is discharged as brine and the evaporated steam flows into the connecting tube of another chamber.
A widely used form of filtration involves reverse osmosis (RO) in which high pressure pumps overcome osmotic pressure to diffuse water molecules through selective pores of semi-permeable membranes that prevent passage of larger molecules such as salt, organic compounds and other solutes. Such membranes are typically layered with other materials that provide channels for separated flow of product water and brine, and are deployed inside pressure tubes in a spiral configuration during operation. Generally, feed water is only partially pumped through a membrane to provide product water while unfiltered feed water is used to flush away brine.
When solutes and other impurities accumulate inside the pressure tubes, the membranes may become fouled and must be cleaned or replaced by removal from the pressure tubes or other process. In order to reduce or minimize fouling of membranes, feed water often undergoes pretreatment in beach wells, sediment filters, carbon filtration, chemical treatment or other methods. The purity of product water may also be improved by re-pumping filtered feed water through membranes in multiple passes.
Another form of filtration utilizes electrodialysis (ED) by applying direct electrical current to feed water within a container having a stack of ion-exchange membranes. Positive sodium ions produced by dissolved salt molecules pass through cation membranes while negative chloride ions pass through anion membranes to convert the feed water into separate streams of brine and filtered water. This process is generally limited to low concentrations of saline water and is subject to fouling of membranes. In order to alleviate fouling, the electrical current may be reversed in a process known as electrodialysis reversal (EDR).
Freezing methods have also been used for desalination. Common techniques generally involve pumping a refrigerant, such as butane or carbon dioxide, directly into feed water within a crystallization unit to produce a slurry of brine and ice crystals. The slurry is then pumped to a wash unit in order to separate ice crystals from brine by using a liquid, such as freshwater, to wash the brine from the ice crystals. To facilitate separation, another liquid such as oil may be added in the wash unit or a separation column. Ice crystals are then transferred to a melter unit where they are melted to provide product water.
In some variations, slurry may be produced by vacuum, eutectic or clathrate processes. Vacuum freezing typically involves spraying feed water into a vacuum chamber where partial vaporization occurs and ice crystals are formed when the water loses heat. The ice crystals then mix with the remaining feed water to produce a slurry. Eutectic freezing generally facilitates separation of brine from ice crystals by forming salt crystals at a temperature where both salt and ice crystals may be formed. Salt crystals are more easily washed away from ice crystals than dissolved salt, which frequently becomes trapped during ice crystal formation. Clathrate freezing may also be used to facilitate separation of brine from ice crystals by using hydrocarbons such as methane, ethane or butane to form hydrates that block salt from becoming trapped inside molecular cages of ice.
Other freezing techniques have used heat exchangers such as tubes, drums or plates for progressive growth of ice crystals along a surface on the opposite side of a wall containing a refrigerant such as ethylene glycol. As an example, feed water may be pumped into a crystallization chamber, such as a tube or drum, that is immersed in a coolant bath so that ice crystals may grow along the internal surface of the crystallization chamber. The crystallization chamber is then removed from the coolant bath so that the ice crystals may be melted to provide product water. Conversely, a refrigerant may be pumped into a tube that is immersed in a tank of feed water so that ice crystals may grow along the external surface of the tube. The tube is then removed from the tank along with the ice, and product water is produced by melting the ice.